The present invention relates to a scroll compressor whose rotational speed is controlled in a wide range from a low speed region to a high speed region, and, more particularly, to an oil supply system for supplying oil to bearings for supporting a crankshaft of the scroll compressor.
In a scroll compressor, a fixed scroll member with a spiral wrap and an orbiting scroll member with a like spiral wrap are engaged with each other and there is a crankshaft for make an orbiting motion of the orbiting scroll member relative to the fixed scroll in order to compress and discharge a fluid. A structure for supporting the crankshaft and an oil supply system for the support structure are shown, for example, in U.S. Pat. No. 4,551,082 in which the crankshaft is supported by a first sliding or plane bearing provided between a crank portion of the crankshaft and the orbiting scroll, a second sliding or ball and roller bearing provided between a shaft portion of the crankshaft and a frame, and a third sliding bearing. According to this system, the oil supply to the bearings is performed such that a centrifugal force is applied to lubricant oil by an eccentric oil path formed through the crankshaft, so as to supply the lubricant to the third bearing The lubricant is supplied to the second bearing by force of, in addition to the centrifugal force generated by the rotation of the crankshaft, a differential pressure between a discharge pressure and a pressure which is intermediate between the discharge pressure and a suction pressure and is applied to the back surface of the orbiting scroll member.
Also, another oil supply method for supplying lubricant to the bearings is disclosed in U.S. Pat. No. 4,462,772 in which the oil supply to the first and second bearings is performed by the differential pressure between the discharge pressure and the intermediate pressure, and the oil supply to the third bearing is performed through a branched pipe by utilizing the action of the centrifugal force of the crankshaft.
In the above-described prior art, the oil supply amount for the respective bearings is determined on the assumption that the scroll compressor be used under a rated condition at a constant rotational speed. On the other hand, there are recent demands that a scroll compressor be operated over a wide range from a low speed to a high speed responsive to the load condition of a refrigeration cycle in which the scroll compressor is incorporated. If the rotational speed of the conventional scroll compressor described above is controlled on such operation condition, the centrifugal force for oil supply is reduced, in particular, in a low speed region. It would be impossible, therefore, to keep a sufficient oil feed, which would lead to a damage of the bearings.
Additionally, since the oil supply by force of the differential pressure as described above is effected in the midway of the second bearing, the space between the second bearing and the third bearing has to be kept under the discharge pressure. It is therefore necessary to provide a spiral groove or a discharge port for discharging the lubricant. This would need large machining work.